Natural killer T (NKT) cells are a subset of T lymphocytes that express surface markers common to both conventional T cells and NK cells. The invariant VA14 T cell receptor of NKT cells recognizes glycolipid antigens bound with the MHC class I-like molecule CD1d. Although the precise immunological function of NKT cells remains unknown, these cells have been implicated in protective immune responses against pathogens and tumors, as well as in the regulation of autoimmune responses. The long-term goal of this proposal is to develop strategies for modulating adaptive immune responses, including autoimmune responses, through activation of NKT cells. Prior work conducted in the P.I.'s laboratory has shown that ligand-specific activation of NKT cells with the glycolipid antigen alpha-galactosylceramide (alpha-GalCer) can polarize adaptive immune responses for production of T helper (Th) type 2 cytokines. These findings suggest that alpha-GalCer may be of therapeutic value for modulating autoimmune responses. Indeed, our preliminary results have revealed that alpha-GalCer can prevent the development of type I diabetes in non-obese diabetic (NOD) mice. The central hypothesis of this proposal is that ligand-activated NKT cells modulate autoimmune responses by producing cytokines that promote the generation of regulatory T cells. We will test this hypothesis, using NOD mice as an experimental model. Aim 1 will investigate mechanisms by which ligand-activated NKT cells modulate autoimmune diabetes in NOD mice. Aim 2 will evaluate the role of CD1d-restricted NKT cells for tolerance induced by blockade of costimulatory T cell activation signals. Aim 3 will determine whether structural analogues of alpha-GalCer can selectively activate distinct adaptive immune responses in vivo. These studies will provide a better understanding of the immunological mechanisms that result in autoimmunity, and may lead to the development of novel agents for treatment of type I diabetes and other autoimmune diseases.